Localization of wheat-germ agglutinin in developing wheat embryos and those cultured in abscisic acid

An embryo-specific protein of barley (Hordeum vulgare)

An immunological approach has been used to identify embryo-specific products that can be used as molecular markers of embryogenesis. Immunoadsorption of antisera to remove antigens common to embryos, meristematic cells and callus, revealed one major embryo-specific antigen, a polypeptide of 17 kDa. The antigen appeared at mid-stages of zygotic embryo formation and remained at similar levels up to six days post-germination of the seedling. The polypeptide could not be detected by protein staining, suggesting it is a non-abundant product. Appearance of the antigen could be induced by culture of zygotic embryos in vitro on abscisic acid (1 microM) or mannitol (9% mass/vol.). Cross-reactive products of near-identical molecular mass were observed in embryos of wheat, rye and oats but not distantly related cereals, nor embryos from dicotyledonous species. The timing of the appearance of the antigen was different in embryos formed from microspores during anther culture in vitro. In the cultured material, the 17-kDa polypeptide preceded the appearance of morphologically distinct embryonic structure.

Abscisic acid enhances the transcription of wheat-germ agglutinin mRNA without altering its tissue-specific expression

We used RNA gel-blot analysis, in-situ hybridization, and nuclear run-on transcription to examine the effects of exogenous abscisic acid (ABA) on the spatial distribution of mRNA for the lectin wheat-germ agglutinin (WGA) in developing wheat (Triticum aestivum L. cv. Marshall) embryos and seedlings. When analyzed by RNA gel blots, both developing embryos and seedlings exhibited higher steady-state levels of WGA mRNA after ABA treatment. As determined by in-situ hybridization, incubation of developing embryos in 0.1 mM ABA resulted in accumulation of WGA mRNA in the epidermal and subepidermal cell layers of the radicle and seminal roots and throughout the rootcap and coleorhiza. This spatial distribution was identical to that in control embryos. Nuclear run-on transcription assays indicated that at least part of this increase is attributable to transcriptional induction. Thus, exogenous ABA is capable of inducing increased WGA mRNA accumulation only in cells where it is expressed during normal embryogenesis. When seeds were germinated in the absence of ABA, WGA mRNA was detected only in the rootcap. In contrast, seeds imbibed and germinated in the presence of ABA for 3 d exhibited a spatial distribution of WGA mRNA similar to that observed in developing embryos treated with ABA. In contrast, when ABA was added to 3-d-old seedlings, WGA mRNA was not detected in regions of the root beyond the rootcap. We conclude that exogenous ABA, when applied continuously from imbibition, causes retention of the embryo-specific pattern of WGA mRNA distribution and that the spatial pattern of WGA mRNA expression in roots does not change when ABA is added after germination.

Cell-type-specific expression of a wheat-germ agglutinin gene in embryos and young seedlings of Triticum aestivum

We have investigated the levels of cell-specific expression of wheat-germ agglutinin (WGA) during the development of embryos and in 3-d-old seedlings. Southern blot analysis of genomic DNA derived from hexaploid and diploid wheat (Triticum) species indicates that the isolated cDNA (complementary DNA) clone is specific for isolectin B (WGA-B). Specific accumulation of transcript for WGA-B was determined by RNA blot analysis and in-situ hybridization. The WGA-B mRNA increased tenfold during embryogenesis between 10 and 40 d post-anthesis and then declined again as the seed reached maturity. The root tips of 3-d-old seedlings contained approximately threefold more WGA-B mRNA than mature seeds. In-situ hybridization experiments showed that during embryogenesis, WGA-B mRNA was present only in the epidermal layers of the radicle and the coleorhiza, while in 3-d-old seedlings it was found in the root-cap cells. To analyze de-novo transcription of WGA-B mRNA a new variation of in-situ hybridization was developed. RNAs from developing embryos and seedlings were labeled with [(3)H]uridine and then hybridized in-situ with unlabeled antisense and sense transcripts. These experiments demonstrated that accumulation of WGA-B mRNA in embryos and 3-d-old seedlings resulted, at least partially, from de-novo transcription. All cells containing WGA-B mRNA also contained WGA, as shown by immunocytochemistry.

Wheat-germ agglutinin is synthesized as a glycosylated precursor

The biosynthesis and processing of wheat-germ agglutinin (WGA) were studied in developing wheat (Triticum aestivum L. cv. Marshall) embryos using pulse-chase labeling, subcellular fractionation and immunocytochemistry. A substantial amount of newly synthesized WGA was organelle-associated. Isolation of WGA on affinity columns of immobilized N-acetylglucosamine indicated that it was present in a dimeric form. When extracts from embryos pulse-labeled with [(35)S]cysteine were fractionated on an isopycnic sucrose gradient, radioactivity incorporated into WGA was detected at a position coincident with the endoplasmic reticulum (ER) marker enzyme NADH-cytochromec reductase. The WGA in the ER could be slowly chased into the soluble, vacuolar fraction, with a half-life of approx. 8 h. Immunolocalization studies demonstrated the accumulation and distribution of WGA throughout the vacuoles.Four forms of the WGA monomer were characterized using immunoaffinity purification and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In-vitro translation of polyadenylated RNA isolated from developing wheat embryos produced a polypeptide with Mr 21 000. In-vivo labeling of embryos with radioactive amino acids resulted in the formation of a polypeptide of Mr 23 000 and the mature monomer of Mr 18000. When [(3)H]mannose was used in labeling studies, only the polypeptide of Mr 23 000 was detected. In-vivo labeling in the presence of tunicamycin yielded an additional polypeptide of Mr 20 000. These results indicate that WGA is cotranslationally processed by the removal of a signal peptide and the addition of a glycan, presumably at the carboxy-terminus (N.V. Raikhel and T.A. Wilkins, 1987, Proc. Natl. Acad. Sci. USA 84, 6745-6749). The glycosylated precursor of WGA is post-translationally processed to the mature form by the removal of a carboxyl-terminal glycopeptide.

Cloning and characterization of a cDNA encoding a mRNA rapidly-induced by ABA in barley aleurone layers

Abscisic acid (ABA) inhibits the gibberellic acid induced synthesis of α-amylase in barley aleurone layers, yet ABA itself induces more than a dozen polypeptides (Lin & Ho, Plant Physiol 82: 289-297, 1986). As part of our effort to elucidate the molecular action of ABA in barley aleurone layers, we have isolated and characterized an ABA-induced cDNA clone, pHV A1. This cDNA clone hybridizes to an RNA species of approximately 1.1 kb from ABA-treated barley aleurone layers. The level of this mRNA is tripled within 40 minutes after ABA treatment, reaches a peak at 8-12 h, and is present up to 48 h. The induction of this mRNA responds to concentrations of ABA as low as 10(-9) M, but higher ABA concentrations induce higher expression of this mRNA. The products of hybrid-select translation and in vitro transcription/translation with pHV A1 comigrate on SDS gel as a 27 kDa polypeptide. However, the sequence of pHV A1 indicates that it has an open reading frame encoding a 22 kDa protein. This size discrepancy is probably due to the high content of the basic amino acid, lysine. This notion has been confirmed by two-dimensional gel electrophoresis showing that this polypeptide is one of the most basic proteins in ABA-treated barley aleurone layers. The deduced amino acid sequence of pHV A1 contains nine imperfect repeats 11 amino acids long which share homology with cotton Lea 7 protein (Baker, Steele & Dure, Plant Mol Biol, in press). The identity and function of the encoded product of pHV A1 is under investigation.

Wheat germ agglutinin accumulation in coleoptiles of different genotypes of wheat. Localization by monoclonal antibodies

We have produced a library of 18 monoclonal antibodies (mABs) against wheat germ agglutinin (WGA). It was difficult to establish antibody-producing hybridomas when soluble WGA was used for immunization. The frequency of specific hybridomas was increased, however, by injecting mice with insoluble antigen-antibody complex.We distinguished groups of mABs that are especially efficient for particular immunoassays. One group (mABs 005, 006, 007, 009, 011, 014, 015, 016, 017, 018, 019) strongly immunostains denatured antigen on electroblots of sodium dodecyl sulfate polyacrylamide gels. A second group (all mABs except 012) shows high activity for WGA when native protein is analyzed by enzyme-linked immunosorbent assay. The third group (mABs 002, 005, 008, 009, 010, 011, 014, 016, 018, 019) works well for immunocytochemistry.We used the mABs to localize WGA in wheat varieties of various ploidy and with different ancestral wheat genomes. Whereas lectin is detected in the coleoptile of varieties with hexaploid and DD and SS genomes, WGA is absent in the coleoptile of the diploid Triticum monococcum (AA). Lectin accumulates in the coleoptile of mature embryos of T. monococcum, however, when they are treated with abscisic acid.